1. Field of the Invention
This invention relates to the field of data communications and specifically to a serial data bus system which provides for inter-device communications on a three wire serial data bus. Data communication bus structures of this type may be utilized in vehicular communication systems which employ one or more peripheral devices, in conjunction with a controller, to provide both multi-user and multi-function access to the vehicular communication system.
2. Description of the Prior Art
Early vehicular communication systems appeared in the form of simple "car telephone" systems which were capable of operating on a single frequency. The operating frequency of this type of system was manually controlled either by a system dispatcher or the user in a vehicle. A system of this type allowed a single user with a single handset to communicate with a dispatcher or base site. A more advanced vehicular communications system is described in U.S. Pat. No. 3,351,714 entitled "Mobile Radio Telephone Apparatus" by R. C. Kunzelman et.al., issued Nov. 7, 1964. This type of vehicular communications system provides automatic channel assignment and includes an improved method of signalling the vehicular units, however, this vehicular system is also limited to a single user with a single handset per vehicular unit.
Recently, vehicular communications systems have developed to allow automatic multiple frequency use by the vehicular unit. A vehicular system of this type is refered to as a cellular mobile telephone system and provides for a dramatically increased capacity for users by virtue of the frequency reuse characteristic. One type of cellular mobile communication system is described in U.S. Pat. No. 3,898,390 entitled "Multiple Zone Communications System and Method" by Wells et al., issued Aug. 5, 1975. Cellular mobile telephone vehicular systems have created new applications for vehicular communications wherein several perpherial may be used along with the usual single user handset. Conventional cellular vehicular communication systems operate with a handset and cradle (referred to as a control head) which communicates with the control unit and associated transceiver. The control unit and transceiver are typically located some distance from the control head, usually being located in the trunk of the vehicle. The control unit and control head are coupled through a control cable which unifies the operation of the vehicular communication unit. Conventional control cables utilize as many as eight individual control wires and are adapted for coupling only a single control head and a single control unit and transceiver. One scheme which has reduced the number of conductors required for a control cable is described in U.S. Pat. No. 4,369,516 entitled Self-Clocking Data Transmission System by John P. Byrns, issued Jan. 18, 1983 and assigned to the assignee of the present invention. The self-clocking data bus allows for data transmission between a control unit and control head without additional conductors being required for synchronization and framming of the data. This self-clocking data transmission system can be utilized with a single handset and control unit, however, no provision is made for multiple units which attempt to place data on the data bus simultaneously. Perpherial devices which could be utilized in a system of this type include auto-dialers, hands-free handset controllers, etc. No data communications bus structure exists which provides for multiple prepherial and handset utilization of a common bus structure while further providing arbitration on a three-wire bus type structure, if several units on the serial bus request service simultaneously.